Heating And Cooling Mechanics and Terminology

There are many ways to heat and cool a home, and there’s lots of terms and jargon that is used that can be really confusing when you are trying to figure out the maintenance and care of your system. We’ll try to help you sort all of it out here.

First, Understand Your Home

Most homes are more than a simple box with a roof. Most have different rooms, and often different floors. The challenge becomes, whether you live in Alaska, The Midwest, New England or in the Deep South, to make sure your home, regardless of construction, is comfortable whether it’s -20F or 105F outside.

Just like a person, a home also needs to breathe. The circulation of air, its filtration for dust and particulates, and its humidity make a big difference in the comfort in your home. This is generally referred to as Indoor Air Quality[1], and it’s one of the big factors you need to consider when making decisions about your home’s heating and cooling system.

The whole point of a heating or cooling system is to adjust the temperature in a room to meet the needs of the people living there. The goal should be to provide that temperature adjustment with as much efficiency as possible, using the least amount of fuel or energy to maximize comfort.

A Little More About Humidity

We also need to account for the amount of water, or humidity in the air, as it’s also an important factor to comfort. Too much humidity will make wood furniture feel sticky, cause condensation on your windows, and can lead to the growth of mold, structural damage to your home, and it can attract pests who love moist environments to breed. Too little humidity can damage furniture, shrink floorboards, baseboards and more revealing gaps.[2] It can also dry out skin and nasal passage, and even make people more vulnerable to colds and flu. The Weather Channel has great resources you can check out to learn more about outdoor and indoor air quality and how it can affect your health and the health of your family.[3]

Understanding the jargon- The Basic Terms

When anyone starts talking about heating and cooling systems, the first thing you might hear are terms like tonnage, BTU’s, SEER, and AFUE ratings.[4] This can easily make your head hurt, but we’ll explain them so you can decode what HVAC contractors are talking about.

AFUE – This is a term that refers to the efficiency of a system. It stands for Annual Fuel Utilization Efficiency. This is a rating that reflects how efficiently a gas furnace converts fuel to energy. An AFUE of 90, for example, means that approximately 90 percent of the fuel is used to provide warmth to your home, while the remaining 10 percent escapes as exhaust.[5]

It’s important to note that efficiency can be an important factor in the cost of heating your home, but the type of system you have and the fuel source you use have the most impact on your cost. For example, electric heat has an AFUE of 100% because none of it is lost in exhaust, but electric heat can sometimes be much more costly than oil or geothermal systems to operate.

BTU– This simply stands for British Thermal Unit, and it’s a measure of heat energy.[6] One BTU is the amount of energy required to heat one pound of water one degree Fahrenheit. Typically, a air conditioning system might be measured by how many BTU’s they produce, and BTU per hour is the amount of power a system has to cool your home. Higher BTU numbers often mean a higher cost for the system overall.

This brings us to the next big issue, which is Load.

A Load calculation is how you can figure out what kind of air conditioning or heating system is best suited for your living space. By taking the overall square footage of your home and multiplying by 35, you can get an estimate of the number of BTU’s required to adequately heat or cool your home. Adjustments might need to be made for parts of your home that receive a lot of sun (add BTU’s) or are shady (subtract 10% of the BTU’s) and then adjust for the kitchen or laundry area that have appliances that give off a lot of heat. HVAC professionals have a lot of experience in the field, and know how to add and subtract these factors, along with how your home is configured (number of floors, split level, shape of rooms, vaulted ceilings, etc.) to make sure you get a system that will perform well in your home.[7]

SEER is also another term you’ll see used to describe cooling systems. It stands for Seasonal Energy Efficiency Ratio. In the US, this is calculated by the cooling output for a season, measured in BTU’s, divided by the amount of power used per hour to get that level of cooling.[8]

You can use this figure to help calculate the amount of power an unit will use over time, and what your end costs may be. In general, the higher the SEER ratio, the more efficient a system is.

Typical whole house cooling systems are required by the government to have a SEER rating of at least 13, where most window units have a SEER of about 10. To get an Energy Star rating, (A US government rating of energy efficiency), a system must have a SEER of at least 14.5. Starting in 2015, new cooling systems in the southern part of the US must have a SEER rating of 14 or higher.

SEER vs AFUE – Both of these terms deal with a measurement of efficiency however they are typically used to measure efficiencies of different types of heating and cooling equipment. Typically AFUE is used to measure the efficiency of gas/oil furnaces or boilers. Conversely, SEER is a measure used predominately in heat pump systems and ductless systems.

The last basic term you should know is Tons or Tonnage when it refers to air conditioning. It’s not a measure of weight, but the capacity of an A/C system. One ton of air conditioning capacity is equal to the ability to remove 12,000 BTU of heat from your home in one hour.[9] To figure out what sort of capacity is necessary for your home, you should consult this great guide from ASM that has all the calculations necessary, depending on what area of the Country you live in.

Ductwork or No Ductwork

Modern construction usually uses ductwork, installed when the home was built, to deliver hot or cold air to the different parts of your home, like arteries deliver blood to different parts of your body. Heating and cooling systems focus on how to heat or cool air with an energy source, and then distributing that air with fans or blowers as needed.

In many parts of the country, homes were built without internal ductwork. If your home does not have ductwork, heat is often provided through radiators, baseboard heaters, space heaters, stoves or other devices that bring heating or cooling to individual rooms or areas of a home, sometimes distributing that heat using fans or blowers. Let’s talk a little bit about the different kinds of heating and cooling systems used in homes today.

How People Heat Their Home

Let’s start from the most basic heating system and move up to the more recent innovations in home comfort.

Fireplaces – Fireplaces typically burn wood or natural gas to produce heat. They do not, as a rule, have a fan or distribution system for the heat, so they tend to heat one room or one portion of a room rather than a whole home. Because a fireplace is typically vented to the outside, they can often lose more heat than they provide to a home. There are also safety and maintenance concerns to take into account, with maintaining a clean flue and chimney, and safe disposal of ash.

Wood Burning or Pellet Stoves – Wood burning stoves are typically used in more rural areas, and burning wood for heat can be cheaper, in some areas than other energy choices. However, there are also concerns about pollution caused by stoves, so the EPA has stepped in with new regulations, requiring new stoves to be more clean burning than in the past. Pellet stoves are less polluting than wood stoves and offer better temperature control and indoor air quality in general.[10] They burn wood or composite pellets and are typically pretty efficient systems.

Furnace – A furnace is basically a large machine that heats up air, water or steam, and circulates it through your house, either by blowing hot air through ductwork and vents, or by circulating water or steam through pipes to radiators and baseboard heaters to rooms throughout your home.[11]

Furnaces typically work on fossil fuels like natural gas or oil. When the fuel is burned, it heats up coils in a heat exchanger, that then transfers the heat to air which is then distributed through your home via a blower or fan. In a water or steam based system, when the fuel burns, it heats up water or creates steam that is then pumps through the pipes to radiators in the home, which act just like a heat exchanger in as gas furnace, transferring heat to the surrounding air.

The efficiency of turning the fuel into heat leads each furnace of have an AFUE-Annual Fuel Utilization Efficiency rating, and minimum standards of efficiency are regulated through the government.


Glossary of Terms

Air flow – How much air your duct system moves. Air conditioners are generally designed to move about 400 cubic feet per minute (cfm) for each ton of AC capacity. In dry climates, that number will be higher (maybe up to 500 cfm/ton), and in humid climates it’ll be lower (~350 cfm/ton).

Air handler – If you have a split system air conditioner, this is the box that contains the guts of the indoor piece. As its name implies, it contains the blower, but I usually include the heating and cooling components as well (i.e., evaporator coil, supplemental resistance heat, furnace) when I use the term.

Boot – The sheet metal transition piece that connects to the duct on one side and has a grille or register on the other.

Checking the charge – Determining how much refrigerant is in the system. When your AC guy puts his gauges on the system, he’s measuring the pressure of the refrigerant to see if you have the right amount.

Compressor – The part of your air conditioner responsible for most of the noise. It sits in the outside part of your AC – the condensing unit – and raises the temperature and pressure of the refrigerant.

Delta T (ΔT) – Temperature difference. If all is working well in your AC, you should get about a 20° F ΔT when the air passes through the cooling stage of the refrigeration cycle.

Ductless mini-split – What the rest of the world uses for air conditioning. It’s a split system heat pump that’s smaller and (usually) has no ducts. The blower and evaporator coil are in the head, which is mounted on a wall or ceiling in the room you’re trying to cool. (See Duct-Free Zone – The Advantages of Mini-Split Heat Pumps.)

EER – The efficiency rating used for window unit ACs and ground source (geothermal) heat pumps: Btu/hr of cooling divided by electricity input in watts, instantaneous.

Geothermal heat pump – The standard heat pump dumps or pulls heat to or from the outside air. A geothermal heat pump dumps or pulls heat to or from the ground or a body of water. Ground source heat pump (GSHP) is a better name for it.

Grille – The type of non-operable cover you see in return vents. (See register.)

Heat pump – An air conditioner that can run in reverse. In summer, it moves heat from inside to outside; in winter, it moves heat from outside to inside.

HVAC – Heating, ventilating, and air conditioning; the general name to cover the whole field. I usually pronounce each letter (H-V-A-C). Chris calls it H-Vac. Either’s fine.

Latent heat – The heat you have to remove from the air to remove the moisture.
Line set – The two refrigerant lines that connect the condensing unit to the evaporator coil. The smaller, hotter, uninsulated copper tube is the liquid line. The larger, colder, insulated tube is the suction line.

Load calculation – Determining how much heat a house gains or loses through the building envelope, from duct losses, and by internal gains (people, appliances…). It’s one part of the HVAC design process. A detailed analysis of your home’s energy needs conducted by your dealer to help determine which comfort system is best for your home.

Hybrid Comfort System – A home comfort system that combines a heat pump with a gas furnace (also available in packaged systems). For areas with colder temperatures, combining electric heating (heat pump) with gas heating (furnace) lets you choose from two fuel sources in order to respond to fluctuations in utility costs.

Home Automation – Anything that gives you remote or automatic control of things around your home, including but not limited to your HVAC, lighting or security system.

HSPF – Heating Seasonal Performance Factor. Refers to the efficiency of the heating mode of heat pumps over an entire heating season: The higher the number, the more efficient the unit.

Energy Star® – A program developed by the Environmental Protection Agency (EPA) in order to reduce the nation’s energy consumption. ENERGY STAR® -qualified heating equipment can be up to 15 percent more efficient than standard models. ENERGY STAR® -qualified cooling equipment can be up to 7 percent more efficient than minimum-standard equipment.

Capacity – The output or producing capability of a piece of cooling or heating equipment. Cooling and heating capacity are normally referred to in BTUs.

BTU – British Thermal Unit. In scientific terms, it represents the amount of energy required to raise one pound of water one degree Fahrenheit. One BTU is the equivalent of the heat given off by a single wooden kitchen match. For your home, it represents the measure of heat given off when fuel is burned for heating, or the measure of heat extracted from your home for cooling.

Tons of air conditioning – Not the weight of the air conditioner but the capacity. One ton of air conditioning capacity is equal to 12,000 Btu/hour. So, a 3 ton air conditioner can remove 36,000 Btu from your home if it runs for an hour. (Actual capacity is not the fixed quantity it might seem, however. See David Butler’s guest post on Manual S equipment selection.)

Ton – A unit of measurement used for determining cooling capacity. One ton is the equivalent of 12,000 BTUs per hour.

Two-stage Heating / Two-stage Cooling – Two-stage heating and cooling is considered to be more efficient, because it operates at a low, energy-saving speed most of the time. However, on days when more heating or cooling is required, it switches to the next stage for maximum comfort.

SEER – Seasonal Energy Efficiency Ratio. This is a rating that measures the cooling efficiency of a heat pump or air conditioner. The higher the number, the more efficient the unit. This efficiency rating for central air conditioners is the Btu/hr of cooling divided by electricity input in watts, averaged over a whole cooling season. 13 SEER is the minimum you can get now.